1. Field of the Invention
Vaccines have been used for the prevention of Marek's disease (MD) in commercial chickens since 1970. There are over 4 billion chickens raised annually in the United States alone. Although vaccination programs have been considered highly effective overall, the poultry industry continues to experience losses due to MD. Given the tendency of MD virus to become more virulent with time coupled with the economic pressures confronting the poultry industry, there is still a strong incentive to develop even more efficacious products that will protect better in the face of early challenge with very virulent field strains. This invention relates to a novel vaccine against MD which does in fact provide superior protection over the existing commercial vaccines.
2. Description of the Prior Art
There are three distinct serotypes of MD virus found in chickens:(1) serotype 1, the oncogenic form responsible for the disease, including high- and low-virulence MD virus and their attenuated variants;(2) serotype 2, a nononcogenic MD virus; and (3) serotype 3, herpesvirus of turkeys (HVT).
The prototype MD vaccine consists of the serotype 3 virus originally isolated from turkeys as reported in Witter et al. I. [Am. J. Vet. Res. 31: 525-538 (1970)]and Okazaki et al., U.S. Patent No. 3,642,574. Its lack of oncogenicity, self-limiting infection, good replication in vivo and in vitro, availability as cell-free and cell-associated preparations, and high protective efficacy have established HVT as a standard for MD vaccines throughout the world. A commonly used strain of HVT is FC126.
Vaccines produced from the naturally avirulent SB-1 strain [Schat et al., J. Natl. Cancer Inst. 60: 1075-1082 (1978) and U.S. Patent No. 4,160,024], an isolate of a serotype 2 MD virus, have been licensed in the United States since 1984. The SB-1 strain is poorly protective against the highly virulent Md5 strain. It is usually used in combination with HVT as a bivalent vaccine since the two viruses together produce greater protection than does either one alone [Schat et al. Avian Pathol. 11: 593-606 (1982); Witter I, Avian Pathol 11: 49-62 (1982)]. This phenomenon has been termed "protective synergism." The SB-1 +HVT bivalent vaccine represents about 18% of the United States market for MD vaccines at present and is considered to be the most efficacious of the various MD products available. However, sporadic losses occur despite its use.
Another MD vaccine produced from strain CVI988 clone C (CVI988/C) has recently been licensed for commercial use in the United States. This vaccine is a mildly virulent serotype 1 MD virus attenuated by serial passage in tissue culture and has been reported by Rispens et al. [Avian Dis. 16: 108-125 (1972)]and deBoer et al. [Avian Dis. 30: 276-283 (1986)].
An experimental vaccine derived from Md11, a very virulent serotype 1 MD field isolate, was reported by Witter I, supra. Md11 was attenuated by 75 serial passages in cell culture, and the resultant vaccine designated Md11/75C. This vaccine has been shown to provide good protection against challenge with Md and most other highly virulent MD viruses tested; but it was less efficacious against challenge with the JM/102W strain, a prototype MD virus effectively protected against by HVT and SB-1 vaccines. Furthermore, its efficacy was consistently lower in chicks with HVT antibody.
Thus, although HVT, SB-1, CVI988/C, and Md11/75C are all effective against certain MD viruses, none of these vaccines protect optimally against all MD challenge viruses in all chickens. In an effort to avert any large-scale outbreaks of MD in the future, the search for improved vaccines has continued.